Many surgical instruments used in the posterior portion of an eye (e.g., a human eye) take the form of a thin tube or needle attached to a handle. The needle may be manufactured from, for example, stainless-steel hypodermic tubing. The tube passes through the wall of the eye and into the interior of the eye, while the handle is used by the surgeon to manipulate the tube from outside of the eye. A variety of tools, fluid, or optical conduits, etc., pass through the tube in order to perform some surgical function at the tip of the tube. Such instruments include, but are not limited to, aspirating cutters, scissors, forceps, fiber-optic illuminators and fiber-optic laser delivery devices. The tube is typically of 20, 23, 25 or 27 gauge (i.e., hypodermic needle gauge). The trend is to use smaller and smaller (i.e., larger gauge number) tubing as current technology evolves to miniaturize the functional elements of the instrument.
Typically, two such instruments are in use at any given time, one in either hand of the surgeon. Often times, one instrument provides illumination through an optical fiber while the other instrument performs some tissue cutting or manipulation function. In addition to using the instrument handles to position the tips of the instruments within the eye, it is common for the instrument handles to be used to “steer” the eye by rotating the eye within the eye socket so as to position a portion of interest of the eye into the field of view of a surgical microscope. This is possible because the points where the instrument tubes pass through the wall of the eye provide a means of applying force (i.e., rotating force) to the eye.
The practice of steering the eye with the instrument handles evolved in a period when most instruments used relatively robust 20 gauge tubing, which is approximately 0.9 mm in diameter. With modern instruments being as small as 27 gauge, which is approximately 0.4 mm in diameter, the practice of steering the eye frequently results in damage to the instrument.
Paralleling the trend to smaller tube diameters, there is also now a trend to place a cannula, a short section of hard tubing, usually with a larger external hub, into the incision through the wall of the eye. However, this practice further increases the likelihood of damage to the instrument, because the forces used by the surgeon to steer the eye are then concentrated at the point where the instrument tube enters the hard cannula. In contrast, steering forces are more evenly distributed on the instrument when the instrument is directly coupled to the softer tissues of the eye wall.